476 RICHARD EVANS. 



fibres. The spicular systems of tlie third class are seldom 

 found in any other position than in the fibres. 



As a rule, the spicules are arranged in the fibres with their 

 axes parallel to one another, and in the deeper parts of the 

 sponge the connecting spicules are rather numerous, and 

 more strongly developed than in the more superficial parts. 

 The connecting spicules are usually the most strongly developed 

 spicules in the whole sponge as regards size, differing, how- 

 ever, only in thickness from the smooth curved amphitornota 

 which constitute the fibres (PI. 37, fig. 3, d — g). Speaking 

 generally, the largest spicules of the first class, together with a 

 few of the second and all the third, form the fibres and the 

 connecting links between them; while the smaller spicules 

 belonging to the first, and nearly all those belonging to the 

 second class, are scattered about irregularly in the meshes 

 between the fibres. The smallest spicules of all appear to be 

 absolutely independent of the skeletal meshwork, and this is the 

 strongest argument that can be adduced in favour of the view 

 that they are microscleres, and not young megascleres. 



The arrangement of the skeleton at large is reticulate. The 

 most prominent feature of the general conformation of the 

 fibres is the way they pass from the surface of fixation of the 

 sponge to the dermal membrane which they support. Along 

 their course from one surface to the other they present a 

 wavy appearance, often dividing and again reuniting, ap- 

 proaching the dermal membrane nearly always at right angles, 

 and in many cases expanding into a kind of brush-like struc- 

 ture which supports that membrane (PI. 38, fig. 6). In some 

 of the largest lobes of the sponge the fibres nearest the centre 

 pursue a straight course, while those furthest from that 

 position curve outwards, so as to form supports to the dermal 

 membrane which covers the flanks of these mound-like 

 elevations. Owing to this arrangement a longitudinal radial 

 section of one of these lobes presents an almost fan-like 

 appearance as regards the skeletal fibres. 



(c) The Spongin. — All the skeletal fibres of this sponge 

 are enclosed in a distinct sheath of spongin, which is greatly 



